Initial documentation of the Invoice: tag.

npo-ledger-cli-tutorial.md

@@ -176,6 +176,25 @@ Some examples of appropriate uses of the `Receipt:` are:
 
 * A pay advice document generated upon payment of an invoice.
 
+#### Invoice Tag
+
+The `Invoice:` tag refers to an actual invoice, either generated by the
+organization or received by the organization.  Typically, this is a document
+that is a request for payment, rather than documenting an actual payment that
+has occurred.  The value of the `Invoice:` tag is always a valid pathname in
+the repository to the document.
+
+Some examples of appropriate uses of the `Invoice:` tag are:
+
+* an actual invoice as sent by a vendor to the organization.
+
+* a request for payment sent by the organization to someone else.
+
+* a reimbursement request submitted by an employee, contractor, or volunteer
+  for expenses they've already incurred and would like the organization to
+  reimburse (e.g., an expense report, requesting for reimbursement of travel
+  expenses).
+
 ### Expense Account Documentation
 
 Each Expense account entries need to be tagged with an `Invoice`, `Receipt`,